Digital or solid state lighting technologies, i.e., illumination based on semiconductor light sources, such as light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs), offer a viable alternative to traditional fluorescent, high-intensity discharge (HID), and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing white light and/or different colors of light, e.g., red, green and blue, as well as a controller or processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038, 6,211,626, and 7,014,336 incorporated herein by reference.
Many lighting applications make use of dimmers. While conventional dimmers work well with incandescent lamps, problems often occur when these dimmers are used with other types of electronic lamps, including compact fluorescent lamp (CR), low voltage halogen lamps using electronic transformers and solid state lighting (SSL) lamps, such as LEDs and OLEDs. Low voltage halogen lamps using electronic transformers, in particular, may be dimmed using special dimmers, such as electric low voltage (ELV) type dimmers or resistive-capacitive (RC) dimmers, which work adequately with loads that have a power factor correction (PFC) circuit at the input.
Dimmers typically have an electronic switch. When the switch is closed, (turned on), a voltage is applied to the output and when the switch is open (turned off), no voltage is applied to the output. Different types of electronic switches may be used in conventional dimmers. For example, triacs may be used, which require a minimum current to stay on. This is the so-called holding current. Low-wattage lamps, such as LED lamps, often fail to draw this minimum current at low dim levels, causing the triac to switch incorrectly causing flicker. Other dimmers use metal-oxide semiconductor field-effect transistors (MOSFETs) or insulated gate bipolar transistors (IGBTs) as the electronic switch. These switches have no minimum current requirement, and thus LED lamps typically work better with these non-triac based dimmers.
Conventional dimmers typically chop a portion of each waveform of the mains voltage signal and pass the remainder of the waveform to the lighting fixture. A leading edge or forward-phase dimmer chops the leading edge of the voltage signal waveform. A trailing edge or reverse-phase dimmer chops the trailing edge of the voltage signal waveform. Electronic loads, such as LED drivers, typically operate better with trailing edge dimmers.
Conventional dimmers have fixed minimum, relatively high RMS output voltages, which limit the minimum light level that can be achieved. Additionally, dimmers may affect the maximum light output, where the maximum light output using a dimmer is lower than the maximum light output without the dimmer. For example, using a typical conventional dimmer, an LED current only can be varied between about 20 percent and about 90 percent of the total current without a dimmer, depending on the dimmer and the lamp. For some conventional lamps, the minimum light level may be as high as 30 percent. There is demand for minimum light levels in the range of 5 percent or lower, as well as for maxim light levels closer to 100 percent.